Why is my carbon monoxide alarm going off when the fireplace is burning?
The alarm is most likely detecting carbon monoxide from the fire that is not leaving through the chimney. Common causes are a cracked or damaged liner, a blocked or partly closed flue, or air being pulled back down the chimney. Stop using the fireplace until a chimney technician has looked at it.
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If anyone has a headache, dizziness or nausea, get everyone outside into fresh air and call 911. Otherwise, let the fire die down, ventilate the room, and treat the alarm as real rather than a nuisance.
Why it happens
A working chimney carries smoke and gases up and out. Carbon monoxide is one of those gases. If the flue is blocked, the gases have nowhere to go and can spill into the room. If the liner is cracked or has gaps, gases and heat can escape into the chimney structure and the walls around it. From there they can find their way into living space, sometimes without any visible smoke.
Draft problems can do the same thing. A house with strong exhaust fans, a dryer or a very tight envelope can pull air down the chimney instead of letting it rise. A flue that is too large or too small for the appliance, or one that is cold and slow to draw, can also let gases linger. A closed or partly closed damper is another simple cause.
It is also possible the alarm is picking up another source, such as a gas appliance or a vehicle in an attached garage. But an alarm that sounds only while the fireplace is burning points at the fireplace, and the safe approach is to find out for certain before burning again.
What you can check yourself
Look at the damper handle
Check that the damper is fully open. If it is stuck or will not move, do not force it. Leave the fireplace unused and mention it to the technician.
Watch for smoke or odor
Notice whether smoke drifts out of the opening, or whether there is a smoky or acrid smell around the fireplace or in nearby rooms.
Note when the alarm sounds
Does it go off only when the fire is lit, or when a fan, dryer or furnace is also running? Write it down. This helps a technician narrow the cause.
Check the alarms themselves
Make sure the alarm is a carbon monoxide model and look at the date printed on it. Alarms wear out and need replacing, but do not assume a sounding alarm is faulty.
Look at the walls and ceiling near the chimney
From inside the house, look for staining, peeling paint or discoloration around the chimney breast. Do not go on the roof or put anything up the flue.
When to call a technician
Call before the fireplace is used again. A sounding alarm with a fire burning is past the point of looking and listening, especially if you also see smoke in the room, staining near the chimney, or a damper that will not open. After any chimney fire, the flue must be inspected before it is used again.
One Call Chimney is a referral and scheduling service. One call to (866) 576-7943 arranges an independent chimney technician or mason. They inspect the chimney, give you the price, and start work only once you approve it. The estimate is free. Whether any repair needs a permit depends on your city or county, so check with your local building department.
What the technician will do
- Camera inspection firstA camera goes through the flue to find cracks, gaps, blockages or damage. Ask to see the footage and have the findings explained.
- Match the flue to the applianceSize, material and termination of the flue are checked against the fireplace or stove it serves. A mismatch can cause poor draft.
- Relining where the liner is damagedThat may mean a stainless steel liner sized to the appliance and insulated where required, or a cast-in-place lining where appropriate.
- Cap and crown addressed at the same timeThe top of the chimney is checked and repaired alongside the liner so the new work is protected.
Related questions
- Can a cracked chimney liner let carbon monoxide into the house?
- Yes. A damaged liner lets heat and carbon monoxide reach the structure of the house instead of leaving through the top. That is why the fireplace should not be used until the liner has been verified.
- Is it safe to keep using the fireplace if the alarm stops?
- No. An alarm that quiets down after the fire dies does not mean the cause is gone. Wait until a technician has inspected the flue before burning again.
- How will I know if the liner is the problem?
- A camera inspection shows the inside of the flue, so cracks, gaps and blockages can be seen directly. Ask the technician to show you the footage and explain what it means in writing.
- What decides what relining will cost?
- The main factors are the chimney's height and access, what the camera finds, the liner material and size needed for the appliance, and whether the cap, crown or flashing also need work. The free estimate sets the price before any work begins.
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